In such belt tensioners standardly used in motor vehicles at present, the retraction device is actuated by a spiral spring, which has a design-based initial tension when the belt is retracted, and is increasingly tensioned according to its spring characteristic as the belt is pulled out.
The extraction lock operates with mechanical catch members. On one hand, these are controlled by inertia members, which are displaced from a position ineffective for the catch bolt to a position effective for the catch bolt, when forces acting on the vehicle body produce a body acceleration or deceleration exceeding a low threshold value. On the other hand, centrifugal members are deflected from a position ineffective for the catch bolt to a position effective for the catch bolt when a reel used for receiving the belt is rotated in the extraction direction of the belt at an angular acceleration exceeding a threshold value or in a jerky manner. This ensures that the belt is reliably restrained from being pulled out (further) in driving situations tending to be dangerous, or even in crashes.
In view of the fact that drivers and/or, in particular, passengers of a vehicle may at least temporarily assume a position outside of the normal or desired sitting position, the belt tensioners, at least those in higher-quality vehicles, are assigned irreversible tensioning devices, which typically function in a pyrotechnical manner and are triggered when a corresponding sensory system “reports” a collision of the vehicle or triggers an airbag present in the vehicle. These irreversible belt tensioners are used to shorten the seat belt with great force. In this manner, any slack in the belt against the body of the occupant, as well as at or on the reel of the belt, is eliminated, and optimal safety for the respective occupant from possible secondary collisions is also ensured after a previously triggered airbag relaxes. In any case, the occupant is kept away as much as possible from hard structural members of the vehicle body.
The irreversible tensioning device may be positioned at a limit stop of the belt, on the seat-belt buckle, or on the belt retractor.
To improve comfort, German Published Patent Application No. 39 38 081 describes assigning an electric motor to the spring provided for actuating the retraction device, in order to adjust the relatively stationary retainer of the return spring. In this manner, the belt tension may be changed, and in particular, the tension of the belt remains low and the wear comfort is improved accordingly, even when the belt is pulled out very far, as is necessary in the case of above-average height or corpulence of the occupant. As soon as the belt is retracted for winding it up, the relatively stationary retainer is returned by the above-mentioned motor to a starting position, so that the belt may be wound up reliably.
Similar arrangements are the subject matter of German Published Patent Application Nos. 41 12 620 and 195 01 076.
In German Published Patent Application No. 100 13 870, it is described to reversibly increase the belt tension when a sensory system signals a dangerous driving condition. This prepares the belt for a possible crash of the vehicle.
It is an object of the present invention to improve the tightening of the belt from the aspects of both safety and comfort.